"In interacting with the environment,
with others, and with the artifacts of technology, people form internal, mental
models of themselves and of the things with which they are interacting. These
models provide predictive and explanatory power for understanding the
interaction."

-Norman (in Gentner &
Stevens, 1983)

Overview:

In Kenneth Craik
's words, mental models are representations in the mind of real or imaginary
situations. Conceptually, the mind constructs a small scale model of reality and
uses it to reason, to underlie explanations and to anticipate events. These
models can be constructed from perception, imagination, or interpretation of
discourse. A mental model represents explicitly what is true, but not what is
false. The greater number of mental models a task suggests, and the greater the
complexity of every model, the poorer performance is.

These models are
more than just pictures or images, sometimes the model itself cannot be
visualized or the image of the model depends on underlying models. Models can
also represent abstract notions like negation or ownership which are impossible
to visualize.

The theory of
mental models demolished an assumption which was until then prevalent in
psychological theories of reasoning: that humans employ a kind of mental logic,
which is similar to the propositional logic employed by logicians, when making
inferences about the world.

Scope/Application:

Mental
models scope is wide enough to range from naive models of natural phenomena such
as electricity to instructional models of scientific concepts used in teaching.
Since human thought is involved in every day's activities,mental models are
applicable to almost every human interaction with nature, devices, and even
interaction with other individuals.

The theory of
knowledge representation and mental models is applicable in designing every
day's things. By knowing what users know about the system and how they can infer
the system functionality from the provided interface, it will be possible to
predict and improve the learning curve as well as users errors and the ease of
use of that system and finally to design interfaces that support the acquisition
of appropriate user model.

Examples:

As an example of
mental models and their influence in every day's interaction, let's take a look
at the sketch by M.C. Escher shown below.

Click on the thumbnail above for the full size version

At a
first glance, the drawing looks perfectly normal ... As you focus on the
structure inside the red circle, something starts to feel confusing. For a
while, it's hard to figure out the illogic part of it since the walking men on
the top direct the eye through the ascending and descending stairs which, as
part seems natural but the mind starts to refuse some concept about what is
perceived. The reason behind this confusion is that the mental model of
doing an action stairs on stairs leads to a different state (going upstairs
brings you to a higher level and going downstairs brings you to a lower level)
and since this concept is violated in the above model, the mind starts having
the confusion and we find words popping out "something's wrong...", "this
doesn't make sense",etc...

Another example
would be considering users making the transition to Windows 95. Mac users were
used to the concept of a recycle bin sitting on their desktop. These users did
not have problems with the newly introduced recycle bin in that version of
windows since they had already built and experienced the right mental model for
it. In contrast, users making the transition from older versions of windows did
not have that model since it was not present in early versions of windows.

Principles:

Knowledge representation:

Knowledge is stored in in memory in a highly organized fashion. It usually takes
the mind a fraction of a second to answer some question such as what is the
capital of Paris or who is the president of the United States .Through this
document, we will refer to the two expressions "knowledge representation" and
"mental representations" as being synonyms.

figure 1 :
Johnson-Laird proposition of the three types of mental representations

Let's take an
example to state the nature of arbitrariness of language like representation
versus the isomorphism of picture representation and then see how mental model
falls somewhere between those two representations. Children who have never seen
an elephant before would not necessarily recognize an elephant in front of them
when all the information stored in their brain about it is the description their
mothers provided them with. The mapping in this case is totally arbitrary.
Meanwhile, a child who has already seen a picture of an elephant would probably
recognize it in real life. Picture like representation is isomorphic mapping
from the mental model to the real life phenomenon. Furthermore this
representation preserves features and relationships.

Although mental
imagery is a rich representation, strong claims exist that prepositional
representation is the basis for mental models. Johnson Laird (1983) described
them as mental echo of the real world information encoded in verbal format. This
representation is mapped to mental models through procedural semantics. During
this process, the mind acquires information about the new phenomena and searched
previously stored models for matching semantics. If no model was found, a
new model will be constructed and stored with the relevant semantics .

The theory of
mental models was formulated in the early 40's by Kenneth Craik. He sought to
provide a general explanation of the human thought based on the assertion that
humans represent the world they interact with through mental models.
Johnson Laird (1983) based his theory on Craik's assumption stating that
an individual holds a working model of a certain phenomenon in order to
understand it. Mental models are not necessarily a visual representation of the
real life case, neither they are more complex representations. Laird
argues that the abundance of representation details does not imply its
usefulness. The only constraint for a mental model is that it has a similar
structure to the phenomenon it represents. An ideal mental model explains all
the aspects of the phenomenon the individual is interacting with. A
referentially isomorphic mapping exists between the mental model and the real
one.

The mental models
theory demolished the old psychological theories assumptions that human beings
use some kind of prepositional logic to make inferences about the world. Laird
tries in his new theory to unravel the puzzling facts discovered by empirical
observations that while individuals are capable of logical inference, yet the
logical reasoning rules cannot apply on the decisions and the answers they
deliver. People tend to solve problems by using prior information and knowledge
about similar problems if a similarity exists between the structure of the two
models.

Structural and Functional
mental models:

In the early 1980's, two basic mental models were identified, which are
Structural and functional models. Structural models define
facts the user has about how a certain system works. Its basic advantage is that
the knowledge of how a device or system works can predict the effect of any
possible sequence of actions, meanwhile constructing such a model in mind
involves a great deal of effort .On the other hand, functional models, also
called task-action mapping models, are procedural knowledge about how to
use the system. The main advantage of functional models is that they can be
constructed from existing knowledge about a similar domain or system.
Structural models are context free while functional models are context sensitive.

Applicability in HCI :

From an HCI
perspective, users form mental models by interacting with a certain computer
system. The content and structure of mental models are influenced by selecting
which information about a certain system is presented to the user and how it is
presented. The interpretation of these models specifies how users interact with
that system. Some major questions in this domain arise such as : To what extent
the form of representation used in the interface affects the way the user solves
a certain problem? Furthermore, Is it possible to develop interfaces that
facilitate problem solving and support creativity? Does a graphical programming
environment support innovation because it provides information in a format that
is closer to the user's mental representation of the problem?

Mental Models in HCI:

Several theories
exist relating different models of users, designers and systems. They proposed
four basic models of models that affect the way users interact with a system
which are User's model of the systemwhich is the model
constructed at the users' side through their interaction with the target system,

the system's model of the user
which is the model constructed inside the system as it runs through different
sources of information such as profiles, user settings, logs, and even errors.
The third model is the conceptual model which is an accurate and
consistent representation of the target system held by the designer or an expert
user, and the last model is the designer's model of the user's model
which is basically constructed before the system exists by looking at similar
systems or prototype or by cognitive models or task analysis.

Several factors influence the way these models are built and maintained. At the
users' side : their physical and sensory abilities, their previous experience
dealing with similar systems, their domain knowledge and finally ergonomics
and environments in which users live.

At the designers'
side, the need is to influence the user's model to perceive the conceptual model
underlying the relevant aspects of the system. This can be accomplished using
metaphor, graphics, icons, language, documentations and tutorials. It is
important that all these materials collaborate together to encourage the same
model.

Example:

As an
example of how users build mental models, let's take an example of a windows
typical user exposed to a Unix environment for the first time and as a task in
hand, he has to type a document on Emacs as opposed to his favorite windows text
editor. The user makes a typo and without hesitating presses his fingers on the
Control and the Z buttons since these are the keys he always used as a keyboard
shortcut for UNDO command. The user gets frustrated as the Emacs editor
completely disappears from the screen and he got back to the Unix prompt with no
single notification message. The fact that the user has been working on windows
builds a mental model for the UNDO command in almost all windows programs and
associates this model with the action of pressing CTRL-Z, not knowing that these
actions will cause a completely different action in Unix environment (which is
running Emacs as a background process and the only way to bring it back is to
type "fg" at the Unix prompt.

Design considerations:

As stated above,
systems should be designed to help users form the correct productive mental
models. Common design methods include the following factors:

Affordance:
Clues provided by certain objects properties on how this object will be used and
manipulated.

Simplicity:
Frequently accessed function should be easily accessible. A simple interface
should be simple and transparent enough for the user to concentrate on the
actual task in hand .

Familiarity:
As mental models are built upon prior knowledge, it's important to use this fact
in designing a system. Relying on the familiarity of a user with an old,
frequently used system gains user trust and help accomplishing a large number of
tasks. Metaphors in user interface design are an example of applying the
familiarity factor within the system.

Availability:
Since recognition is always better than recall, an efficient interface should
always provide cues and visual elements to relieve the user from the memory load
necessary to recall the functionality of the system.

Flexibility:
The user should be able to use any object, in any sequence, at any time.

Feedback:
Complete and continuous feedback from the system through the course of action of
the user. Fast feedback helps assessing the correctness of the sequence of
actions.

Limitations and challenges:

The biggest
limitation on the mental models theory is to capture and validate these models.
Studies have shown that these models are built "on-the-fly", moreover, other
studies have shown that asking a subject about his mental model can modify the
mental model itself (Rogers, et.al.
1992).

"Descriptions
of the formation of mental models rely on a variety of abstract concepts and
processes such as schemas. Consequently, they seem to be an abstraction composed
of abstractions. They are highly subjective. Most studies obtain
descriptions of high-level performance that are difficult to link to the
building blocks of mental structures and processes. Most studies of mental
models also elicit mental model information after formation. Few studies map
specific elements of a stimulus/system (including cues) to mental model
component formation. (Roger, et.al. 1992)

There are marked differences
in the validation criteria accepted by the cognitive science community and
that for HCI practitioners. Cognitive science demands statistically
significant study with strict controls applied from the scientific method.
HCI studies of mental models, and usability in general, are usually less
robust than cognitive science studies. They have a different purpose,
commercial feasibility. Introspection is commonly used. (Rogers, et.al.,
1992)"